Group III nitride compound semiconductor are direct-transition type of semiconductors exhibiting a wide range of emission spectra from UV to red light when used in a device such as a light-emitting device, for example, light-emitting diodes (LEDs) and laser diodes (LDs). In addition, due to their wide band gaps, devices employing the aforementioned semiconductors are expected to exhibit reliable operational characteristics at high temperature as compared with those employing semiconductors of other types, and thus application thereof to transistors such as FETs has been energetically studied. Moreover, because Group III nitride compound semiconductors contain no arsenic (As) as a predominant element, application of Group III nitride compound semiconductors to various semiconductor elements has been expected from the environmental aspect. Generally, these Group III nitride compound semiconductors are formed on a sapphire substrate.